Wordpress - Turn Off Automatic Trash Deletion?

You can do the following:

• Set EMPTY_TRASH_DAYS to a very high number. E.g.: define( 'EMPTY_TRASH_DAYS', 1000000 );

• Or remove the wp_schedule_delete action:

  function my_remove_schedule_delete() {
      remove_action( 'wp_scheduled_delete', 'wp_scheduled_delete' );
  }
  add_action( 'init', 'my_remove_schedule_delete' );
  

My guess is - yes, you are stuck with exactly that.

Checks for EMPTY_TRASH_DAYS seem to be hardcoded as condition for switching interface between trash and delete in several places.